[PATCH v3 13/15] KVM: Handle page fault for private memory

Chao Peng
2021-12-21T15:13:52Z

When a page fault from the secondary page table while the guest is
running happens in a memslot with KVM_MEM_PRIVATE, we need go
different paths for private access and shared access.

- For private access, KVM checks if the page is already allocated in
the memory backend, if yes KVM establishes the mapping, otherwise
exits to userspace to convert a shared page to private one.

- For shared access, KVM also checks if the page is already allocated
in the memory backend, if yes then exit to userspace to convert a
private page to shared one, otherwise it's treated as a traditional
hva-based shared memory, KVM lets existing code to obtain a pfn with
get_user_pages() and establish the mapping.

The above code assume private memory is persistent and pre-allocated in
the memory backend so KVM can use this information as an indicator for
a page is private or shared. The above check is then performed by
calling kvm_memfd_get_pfn() which currently is implemented as a
pagecache search but in theory that can be implemented differently
(i.e. when the page is even not mapped into host pagecache there should
be some different implementation).

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
---
arch/x86/kvm/mmu/mmu.c | 64 +++++++++++++++++++++++++++++++---
arch/x86/kvm/mmu/paging_tmpl.h | 11 ++++--
2 files changed, 68 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index a7006e1ac2d2..7fc29f85313d 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -3156,6 +3156,9 @@ int kvm_mmu_max_mapping_level(struct kvm *kvm,
if (max_level == PG_LEVEL_4K)
return PG_LEVEL_4K;

+ if (kvm_slot_is_private(slot))
+ return max_level;
+
host_level = host_pfn_mapping_level(kvm, gfn, pfn, slot);
return min(host_level, max_level);
}
@@ -4359,7 +4362,50 @@ static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
kvm_vcpu_gfn_to_hva(vcpu, gfn), &arch);
}

-static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault, int *r)
+static bool kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
+ struct kvm_page_fault *fault,
+ bool *is_private_pfn, int *r)
+{
+ int order;
+ int mem_convert_type;
+ struct kvm_memory_slot *slot = fault->slot;
+ long pfn = kvm_memfd_get_pfn(slot, fault->gfn, &order);
+
+ if (kvm_vcpu_is_private_gfn(vcpu, fault->addr >> PAGE_SHIFT)) {
+ if (pfn < 0)
+ mem_convert_type = KVM_EXIT_MEM_MAP_PRIVATE;
+ else {
+ fault->pfn = pfn;
+ if (slot->flags & KVM_MEM_READONLY)
+ fault->map_writable = false;
+ else
+ fault->map_writable = true;
+
+ if (order == 0)
+ fault->max_level = PG_LEVEL_4K;
+ *is_private_pfn = true;
+ *r = RET_PF_FIXED;
+ return true;
+ }
+ } else {
+ if (pfn < 0)
+ return false;
+
+ kvm_memfd_put_pfn(pfn);
+ mem_convert_type = KVM_EXIT_MEM_MAP_SHARED;
+ }
+
+ vcpu->run->exit_reason = KVM_EXIT_MEMORY_ERROR;
+ vcpu->run->mem.type = mem_convert_type;
+ vcpu->run->mem.u.map.gpa = fault->gfn << PAGE_SHIFT;
+ vcpu->run->mem.u.map.size = PAGE_SIZE;
+ fault->pfn = -1;
+ *r = -1;
+ return true;
+}
+
+static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
+ bool *is_private_pfn, int *r)
{
struct kvm_memory_slot *slot = fault->slot;
bool async;
@@ -4400,6 +4446,10 @@ static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
}
}

+ if (kvm_slot_is_private(slot) &&
+ kvm_faultin_pfn_private(vcpu, fault, is_private_pfn, r))
+ return *r == RET_PF_FIXED ? false : true;
+
async = false;
fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, &async,
fault->write, &fault->map_writable,
@@ -4446,6 +4496,7 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
bool is_tdp_mmu_fault = is_tdp_mmu(vcpu->arch.mmu);

unsigned long mmu_seq;
+ bool is_private_pfn = false;
int r;

fault->gfn = kvm_gfn_unalias(vcpu->kvm, fault->addr);
@@ -4465,7 +4516,7 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
mmu_seq = vcpu->kvm->mmu_notifier_seq;
smp_rmb();

- if (kvm_faultin_pfn(vcpu, fault, &r))
+ if (kvm_faultin_pfn(vcpu, fault, &is_private_pfn, &r))
return r;

if (handle_abnormal_pfn(vcpu, fault, ACC_ALL, &r))
@@ -4504,7 +4555,7 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
else
write_lock(&vcpu->kvm->mmu_lock);

- if (is_page_fault_stale(vcpu, fault, mmu_seq))
+ if (!is_private_pfn && is_page_fault_stale(vcpu, fault, mmu_seq))
goto out_unlock;

r = make_mmu_pages_available(vcpu);
@@ -4522,7 +4573,12 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
read_unlock(&vcpu->kvm->mmu_lock);
else
write_unlock(&vcpu->kvm->mmu_lock);
- kvm_release_pfn_clean(fault->pfn);
+
+ if (is_private_pfn)
+ kvm_memfd_put_pfn(fault->pfn);
+ else
+ kvm_release_pfn_clean(fault->pfn);
+
return r;
}

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 6d343a399913..ebd5c923f844 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -842,6 +842,8 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
int r;
unsigned long mmu_seq;
bool is_self_change_mapping;
+ bool is_private_pfn = false;
+

pgprintk("%s: addr %lx err %x\n", __func__, fault->addr, fault->error_code);
WARN_ON_ONCE(fault->is_tdp);
@@ -890,7 +892,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
mmu_seq = vcpu->kvm->mmu_notifier_seq;
smp_rmb();

- if (kvm_faultin_pfn(vcpu, fault, &r))
+ if (kvm_faultin_pfn(vcpu, fault, &is_private_pfn, &r))
return r;

if (handle_abnormal_pfn(vcpu, fault, walker.pte_access, &r))
@@ -918,7 +920,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
r = RET_PF_RETRY;
write_lock(&vcpu->kvm->mmu_lock);

- if (is_page_fault_stale(vcpu, fault, mmu_seq))
+ if (!is_private_pfn && is_page_fault_stale(vcpu, fault, mmu_seq))
goto out_unlock;

kvm_mmu_audit(vcpu, AUDIT_PRE_PAGE_FAULT);
@@ -930,7 +932,10 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault

out_unlock:
write_unlock(&vcpu->kvm->mmu_lock);
- kvm_release_pfn_clean(fault->pfn);
+ if (is_private_pfn)
+ kvm_memfd_put_pfn(fault->pfn);
+ else
+ kvm_release_pfn_clean(fault->pfn);
return r;
}

--
2.17.1